An arrangement for remotely controlling an operating table is known which comprises a mobile transmitter and a receiver associated with the operating table, wherein the transmitter has a number of feed-in keys, corresponding to the number of functions of the operating table to be controlled. Said feed-in keys are combined physically to form a feed-in keyboard for feeding-in binary command signals associated with the functions in a 1-of-n code. A frequency generator controllable as to its frequency has a plurality of outputs. By feeding-in a code word corresponding to the respective command signal, and switchable on as a function of the presence of a command signal, the frequency generator outputs are controlled. A transmission converter is fed by the frequency generator and transmits frequency signals corresponding to the command signals. A receiver has a reception converter and means for selective amplification of the frequency signals received and for their reconversion into the command signals. In this case, by the actuation of a feed-in key, the frequency generator is constructed as a free-swinging oscillator and is connected to a capacitor, while the capacitors which can be switched on by means of different keys exhibit different values so that a different frequency is associated with each feed-in key. With this arrangement, due to the necessary build-up processes and to the echo effects which occur, an accurate evaluation of the frequency impulses transmitted as to their duration is impossible. Furthermore, the arrangement is very highly sensitive to interference. But, particularly in hospitals, where remote control arrangements for medical appliances are frequently used, there exists a large number of such interference sources. For example, if the ultrasonic range is chosen for the transmission of the frequency signals, then interference signals may originate from ultrasonic washing machines for instruments, ultrasonically operated surgical hand washing installations, high-frequency surgical appliances, ultrasonic diagnostic appliances or ultrasonic bone welding appliances. Experience also shows that ultrasonic components occur in many resonance phenomena, e.g., in wind noises in flue ducts or in telephone installations. The susceptibility to interference is particularly serious when, e.g., in a hospital with a plurality of operating theaters, the respective operating tables are required to be operated by means of similar remote control arrangements, so that each of these arrangements then acts as an interference transmitter for at least the remote control arrangements used in the adjacent rooms.
A remote control arrangement similar to the aforementioned type is also known for television receiving sets, wherein the frequency generator is constructed to generate a group frequency signal in addition to the frequency signals corresponding to the command signals as a function of the feeding-in of an additional group code word. The transmitter has an impulse generator which can be set in action as a function of the presence of a command signal. The output impulses generated by the impulse generator are a function of the group code word fed into the frequency generator instead of the code word corresponding to the respective command signal. The receiver has a circuit which controls the emission of the command signals from the alternate reception of a frequency signal corresponding to the command signal and the group frequency signal. Since in this case the command signals become effective in the controlled appliance only when a frequency corresponding to the command signal and the group frequency are received alternately, the freedom from interference is considerably improved. However, any use of such a remote control arrangement for controlling medical appliances is generally made impossible by the fact that the group frequency is fixedly prescribed and cannot be modified without extensive structural modifications to the transmitter. In addition, in many cases, it is not known beforehand what interference frequencies will be encountered at the respective place of use, so that the group frequency cannot be determined at the time of constructing the remote control arrangement. It is, therefore, desirable to be able to adjust the group frequency at the place of use in a simple manner, and this need is enhanced where a plurality of similar remote control arrangements are used at the same location or where further remote control arrangements are added to those already in place.
It is the underlying aim of the invention to produce a remote control arrangement for a medical appliance, which by the transmission of a group frequency alternating with the frequency corresponding to the command signal exhibits improved freedom from interference and wherein the group frequency can be modified easily according to individual requirements.